The high resolution capability of the single beam index cathode ray tube (CRT) is well known. The proliferation of computer terminals and the desire to utilize color CRTs for information display has understandably created an interest in beam index CRTs as color monitors. As discussed in the above-mentioned copending application and patents, index signals of the wide-band type, which literally control the beam on a stripe-by-stripe basis rather than on a line-by-line basis, afford an added dimension in display precision. In a pixel display wherein a group of pixels of information is stored in a computer memory corresponding to a pixel array that is reproducible on the face of a CRT, the index signal is used to trigger the release of red, green and blue (RGB) video data for controlling the current in the CRT beam on a pixel-by-pixel basis. RGB video data applied to the tube is controlled by the index signal, with the result being that the color data is supplied when the beam is over the corresponding color stripe. A "black surround" of inert material between adjacent color phosphor stripes provides a guard band or safety zone to help prevent the electron beam from "spilling over" onto an adjacent color stripe.
In accordance with the above-mentioned related patents and application, the periodicity of the index strips in the CRT is known and once an index strip has been identified, the scanning electron beam is turned off until the next index strip is anticipated, whereupon the beam is again turned on. In a narrow-band system, on the contrary, the index signal is developed for each line from a plurality of "run in strips" situated off screen. Because of these factors, the index beam current, called the index video, may be made quite small in a narrow-band system.
The index signal in a wide-band system is prone to noise in the form of stray photons emitted by the index strips, which generally comprise monochrome phosphors. The stray photons are due to bombardment by "spillover" electrons, that is electrons intended for adjacent color stripes. Consequently, to assure that the wide-band system can find or recognize an index strip, a much larger index video beam current is required than that used in narrow-band systems, which have much better signal to noise ratios. Turning off the electron beam between index strips in the wide-band system enhances the blackness of the picture tube screen, thus improving the contrast of the display.
The present invention further enhances the blackness of the screen during periods of no video data, which in a color monitor supplied with pixel type data, can be quite significant. As is well known, the blackness of the screen is determined by the ambient light level. In a no video condition, any energization of the color stripes is very noticeable. Even the relatively low level index video beam may spill over onto an adjacent color stripe and create undesirable light at the CRT viewing screen. The invention recognizes that under a no video condition, the index strip will be easier "to find" because of the absence of stray photons caused by video beam spill-over. Therefore, when the beam is positioned to strike an index strip and there is no video information, a lower level of index video drive is used. This lower level index video drive results in less spillover of the index video electron beam onto adjacent color phosphor stripes, with consequent minimization of random illumination. Thus the quality of the display is enhanced under a no video condition while the precision in identifying the index strips is maintained under video conditions. This is accomplished without the sacrifice of brightness that would accompany an increase in the guard band.